The present invention relates to telephones, facsimile machines, computer modems, automatic alarm dialers, and similar equipment, and specifically to detection apparatus in such equipment for ascertaining whether a telephone line is in On-Hook or Off-Hook condition, whether there is a ring signal on the line, and whether the voltage on the line in Off-Hook condition is so high that there is risk for overheating of the connected electronic components.
A residential or business user might connect more than one device to one common telephone line. When one of the devices has an auto-dialing function, such as in a facsimile machine or a modem, problems will arise if the telephone line is in use (Off-Hook) when the automatic device attempts to dial a number. The automatic device thus should be able to detect whether the telephone is Off-Hook or On-Hook before dialing is attempted. Many telephone authorities will not allow the use of automatic dialing equipment on their lines unless they contain line state detectors that block dialing attempts when the line is Off-Hook.
A telephone line in On-Hook condition has no current flow, and the line voltage is high, typically 50 V DC. A telephone line in Off-Hook condition sees a current consumption of 10-50 mA DC, and the line voltage is low, about 1-10 V DC. The On-hook/Off-hook line state can thus be detected either by sensing the current in the line, or by sensing the line voltage.
Most of the presently used devices for detecting the On-Hook/Off-Hook state of a telephone line rely on sensing the current in the telephone line, usually by connecting a relay directly in series with the telephone wires. The relay coil must have low resistance, typically max 20 ohm, in order not to cause too much voltage drop in the line, and the relay must close at a current of 10 mA DC or less in the coil. The relay coil must also be shunted by a large bipolar capacitor, typically about 50 .mu.F, to avoid attenuation of high frequency signals. Some telephone authorities specify that the relay must have two coils, each with a shunt capacitor, in order to maintain symmetry on the telephone line. This known detector circuit works well, but the components are expensive and bulky.
The current in the telephone line could also be sensed by means of an opto-coupler instead of a relay, but this introduces a constant voltage threshold of about 1 V in the telephone line, which is not acceptable to many telephone companies, so this method is rarely used.
A detector for On-Hook/Off-Hook state of a telephone line can also be based on sensing of the line voltage instead of line current. Known voltage sensing devices for detection of the On-Hook/Off-Hook state of a telephone line have not been competitive, because telephone authorities require high voltage galvanic insulation from the telephone line, and extremely low loading of the telephone line in On-Hook state. Most of the known devices using voltage sensing are accordingly expensive and bulky.
U.S. Pat. No. 4,647,723 describes a voltage sensing detector for the On-Hook/Off-Hook state of a telephone line. This device needs an amplifier in the voltage sensing circuit, because the low current loading allowed by the telephone authorities on an Off-Hook telephone line can not directly drive an output device providing galvanic separation. The amplifier needs power, which must be galvanically separated from all circuits outside the voltage sensing circuit, so a battery is included as a power source. This makes this detector complicated and expensive, and the battery is a bulky and an inconvenient component.
U.S. Pat. No. 4,220,825 describes another voltage sensing detector for On-Hook/Off-Hook state of a telephone line. This device includes a capacitor in series with a resistor connected across the telephone line via contacts on a relay, so the capacitor is charged to the line voltage. In response to a polling signal, the capacitor, still in series with the resistor, is switched by the relay from the telephone line to the input of a voltage sensing device, and the voltage sensing device determines if the capacitor voltage corresponds to On-Hook or Off-Hook line state. By switching the capacitor/resistor combination between the telephone line and the voltage sensor, the insulation resistance in the relay contacts provide galvanic insulation between the line and the voltage sensor, so there is no problem in providing power to the voltage sensor. But the resistor in series with the capacitor must be low compared to the input resistance of the voltage sensing device, so the resistance can not be large enough to satisfy the loading restrictions imposed by many telephone companies. The galvanic insulation between relay contacts is seldom more than a few hundred volts, which is far below the requirement for several kV insulation by most telephone authorities. Fulfilling this requirement would be impractical with the design described in this reference.
U.S. Pat. No. 4,958,371 describes still another voltage sensing detector for On-Hook/Off-Hook state of a telephone line. This device uses the input circuit of an opto-coupler in series with a zener diode to measure the voltage on the telephone line in response to a polling signal. The opto-coupler provides the required galvanic insulation between the telephone line and the rest of the telephone equipment. The input circuit of an opto-coupler has essentially zero resistance, so this detector will load the telephone line heavily if the line voltage is larger than the zener voltage when the line is polled, which is the indication that the line was in On-Hook state before polling. Such a heavy loading on an On-Hook telephone line is not allowed by telephone authorities, so monitoring the line state with this device will violate their specifications.